The present invention relates generally to the art of assessing the severity of stenosis in a human subject, and more particularly, to an apparatus and method that rapidly assesses the severity of a stenosis using magnetic resonance imaging (MRI) technology.
The narrowing or constriction of vessels carrying blood to the heart is a well-known cause of heart attacks, and gone untreated, can lead to sudden death. In such stenotic vessels, it is known that the region immediately downstream from the constriction is characterized by having rapid flow velocities and/or complex flow patterns. In general, narrowing of blood carrying vessels supplying an organ will ultimately lead to compromised function of the organ in question, at best, and organ failure at worst. Quantitative flow data can readily aid in the diagnosis and management of patients and also help in the basic understanding of disease processes. There are many techniques available for the measurement of blood flow, including imaging based methods using radiographic imaging of contrast agents, both in projection and computed tomography (CT), ultrasound, and nuclear medicine techniques. Radiographic and nuclear medicine techniques require the use of ionizing radiation and/or contrast agents. Some methods involve making assumptions about the flow characteristics which may not necessarily be true in vivo or require knowledge about the cross-sectional area of the vessel or the flow direction.
The functional significance of a stenosis is conventionally determined using Doppler ultrasound to measure the velocity/pressure gradient across the vessel constriction along the axis of flow. The higher the gradient, the more significant the stenosis. However, using Doppler ultrasound is dependent on having an acoustic window allowing the ultrasound beam to insonify the vessel of interest at an angle of incidence as close to zero (i.e., parallel to the vessel) as possible. Furthermore, Doppler ultrasound does not provide the quality of images that are produced using MR technology. Further, ultrasound techniques are difficult to apply in certain situations because of intervening tissues such as bone, excessive fat or air.
Phase contrast magnetic resonance angiography (MRA) is a practical and clinically applicable technique for imaging blood flow. MRI utilizes radio frequency pulses and magnetic field gradients applied to a subject in a strong magnetic field to produce viewable images. When a substance containing nuclei with net nuclear magnetic moment, such as the protons in human tissue, is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field (assumed to be in the z-direction), but precess about the direction of this magnetic field at a characteristic frequency known as the Larmor frequency. If the substance, or tissue, is subjected to a time-varying magnetic field (excitation field B.sub.1) applied at a frequency equal to the Larmor frequency, the net aligned moment, or "longitudinal magnetization", M.sub.Z, may be nutated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated (as the excited spins decays to the ground state) and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
MRA makes use of flow encoding gradient pulses which impart a velocity-dependent phase shift to the transverse magnetization of moving spins while leaving stationary spins unaffected (Moran P. R. A Flow Velocity Zeugmatographic Interlace for NMR Imaging in Humans. Magnetic Resonance Imaging 1982; 1: 197-203). Each phase contrast acquisition generates two images: a magnitude image that is proportional to the proton density of the object and may also be T.sub.1 -weighted, and an image representing the phase of the object. The phase image produced has information only from the moving spins and the signal from stationary tissue is suppressed. Images representing both the average flow over the entire cardiac cycle and at a series of individual points in the cycle have been generated using this technique. The phase contrast MR method produces phase images with intensities that represent the magnitude of the flow velocity and also the direction of flow. Therefore, such images may be used for both qualitative observation of blood flow and quantitative measurement. The practical application of phase contrast MR angiography and venography to the quantitative determination of flow velocity is therefore evident.
It would therefore be desirable to have a method and apparatus for rapid assessment of stenosis severity using MR technology.